The present invention relates to a gas pipe line leak detecting method and apparatus for detecting gas leaks in a gas pipe line.
While various leak detecting methods have been proposed in the past for use with liquid transportation pipe lines, there exist the following great differences between the liquid transportation pipe line and the gas pipe line and therefore the leak detecting methods heretofore proposed for use with liquid transportation lines cannot be applied as such to the gas pipe line.
(a) Firstly, gas is a compressible fluid and therefore the line pack quantity of the gas in the pipe line suffers dynamic variations. As a result, there occur the following phenomena which are not encountered in the liquid pipe line. For instance, a considerable time is required for the effect of a change in the pressure, flow rate or the like at one end of the pipe to appear at the other end. Moreover, this effect arrives in a considerably decreased form at the other end due to its attenuation and diffusion in the course of its proparation through the pipe line. Of course, it is impossible to synchronize the two events and also the effect does not simply appear at the other end with a given time delay.
(b) A change in the pressure, flow rate or the like due to the occurrence of a leak is much smaller than that in the case of a liquid and also the attenuation is so large that it is extremely difficult to distinguish such change from various phenomena which always occur under the ordinary operating conditions. In addition, the propagation time of the change is extremely slow as compared with the case of the liquid. Thus, the detection of a leak in the gas pipe line has been performed in the following ways.
The conventional methods have been mainly of the type which effects the detection of a gas leak by the use of a gas detector including a sensor for directly sensing a leaked gas component. It has been the prevailing practice so that the gas detector is installed at a place having a high probability of a leaked gas floating in the case of a yard, the gas detector is mounted within the valve house in the case of a gas pipe line or a patrol car equipped with the detector makes the round for monitoring purposes.
Also, a report has been made of a method in which a sound generated at a leak by a leaking gas is picked up by a portable microphone.
From the operation standpoint of a gas pipe line, it has been the usual practice so that the pressure is measured at the dispatching base, destination base or midway point of the gas pipe line and compared with its predetermined upper and lower limit values thereby indirectly determining any leak upon detection of an abnormal condition and generally only the detection of large leakage is possible. Also, the predetermined values are frequently dependent on the personal experience of the operator.
However, these conventional methods require much labor for the detection in cases where the leakage gas is detected directly by the sensor or the leakage sound is picked up and there is another disadvantage that the amounts of leakage that can be detected by monitoring the pressure lower limit value are limited to very large amounts of leakage.